The present invention relates to an exhaust gas recirculation system (hereinafter referred to as an EGR system) of an internal combustion engine.
EGR systems, especially outer EGR systems, are well known as systems which can extract a part of the exhaust gas from an exhaust passage of the engine and feed the extracted exhaust gas into an intake passage located downstream or upstream of a throttle valve in an intake system of the engine via a special passage disposed outside of the engine, so as to reduce the NO.sub.x formation in the exhaust gas. A feedback control valve (hereinafter referred to as an EGR valve) for controlling the amount of the exhaust gas fed back to the intake passage is disposed on the special passage. Such EGR valve is generally controlled in accordance with the operating conditions of the engine, especially with the vacuum pressure level in a vacuum port opening into the intake passage at a position adjacent to the throttle valve. In this specification, this vacuum port is an EGR port which opens into the intake passage at a position located upstream of the throttle valve when the throttle valve is in the idling position and which opens into the intake passage located downstream of the throttle valve when the opening degree of the throttle valve exceeds a predetermined degree. Therefore, the actuating pressure level in the EGR port is maintained at the atmospheric pressure level when the throttle valve is in the idling position, and the actuating pressure level is rapidly changed into a high vacuum level when the opening degree of the throttle valve exceeds a predetermined degree. Then, the actuating pressure level in the EGR port gradually approaches to the atmospheric pressure level in accordance with the increasing opening degree of the throttle valve. The EGR valve is controlled so as to open when the absolute value of the vacuum pressure level in the EGR port exceeds a predetermined value. Accordingly, the exhaust gas recirculation is carried out only when the opening degree of the throttle valve is larger than a predetermined degree and the engine is in the low load operating condition.
However, when an engine, on which the above-mentioned exhaust gas recirculation system is mounted, is operating at high altitudes wherein the density of the air is reduced due to decreased atmospheric pressure, some problems in the operation of the engine will occur.
One problem is that a drop in the engine power generally occurs at high altitudes rather than at lower altitudes. To compensate the drop in the engine power, the throttle valve may be opened wider at higher altitudes than at lower altitudes. As a result, the pressure level of the vacuum fed from the EGR port to the EGR valve is lowered to a level corresponding to less than the predetermined value above which the EGR valve is caused to open. Therefore, the EGR valve is closed easily at high altitudes, and thus it becomes impossible to fully achieve the effect of reducing NO.sub.x formation in the exhaust gas.
Another problem is that an error in the ignition timing of the engine will occur at high altitudes. In a conventional engine, since the pressure of the vacuum fed to a vacuum advance device in an ignition system of the engine is decreased at high altitudes, the ignition timing at high altitudes is compensated to advance rather than retard the ignition timing at low altitudes. However, in this case, if the EGR valve is closed, namely, if exhaust gas recirculation control is not effected, at high altitudes as mentioned above, the ignition timing of the engine will be advanced too much. As a result, a drop in the engine power and also knocking in the engine will often occur.